In the field of agriculture and foodstuff, insects cause important damage both at the level of the crops and at the level of the storage of the products.
Generally, a systematic insecticidal treatment is carried out to eliminate the insects.
Though it is relatively easy to determine the nature and the density of insects by means of a mere visual checking of the portion of the crops above the ground, the same does not apply to the products, in particular cereals, when they are stored or transported whether in a warehouse, a silo or a truck, since it is very difficult to detect the degree of infestation inside the batch.
Presently, the analysis is carried out by sieving one or several samples. However, the infestation can be uniform or not, and be developed more or less in depth. Therefore, in order to obtain less random results, samples are taken in the areas which insects are likely to shelter, for example in the areas where a rise in temperature has been observed, which can be a sign for the presence of insects.
If only one insect is discovered in a sample, the treatment of the whole batch is carried out, and if no an insect is discovered in the samples, it frequently occurs that, in case of doubt, one also treats the whole batch. Thus, systematic applications of insecticides are observed, which result into an over-treatment.
The drawback of this process is that the treatments used are carried out based on chemicals, which are often toxic ones.
One distinguishes two types of treatment, the shock treatment and the remanent treatment. The shock treatment is based on the use of dichlorvos, which is classified as toxic, which kills all the insects within 2 or 3 hours, and which, because of its high volatility, almost completely disappears within 48 hours. The remanent treatment uses slowly degrading molecules which allow a remanent action of the insecticide, their effectiveness is spread over a longer time, it takes 72 hours to kill all the insects, and the destruction of new insects is ensured for at least 3 to 4 months.
The use of insecticides is presently a very strong concern with cereal growers, in particular with the processors, who will thus proscribe insecticides in the short run, while asking for insect-free cereals.
There are alternative methods allowing fighting against the infestation by insects, primarily based on a cooling of the stocks by ventilation. These methods can allow limiting the over-treatments, but require, on the other hand, a rigorous follow-up of the stocks and reliable insect-detecting methods.
There have already been provided devices allowing detecting the presence of insects, and eventually their classification as well as their quantification. Most of these devices are seldom used, because they implement detecting means, collecting means and analyzing means the use of which is complex.
Thus, from U.S. Pat. No. 5,646,404 are known an apparatus and a method for quantitatively detecting granivorous insects, said apparatus comprising infrared means capable of detecting the insects contained in a quantity of grains passing though a channel. This device consists, in fact, of an improved sieve, the result of which depends, here too, on the place of sampling.
There are also known detecting methods using means capable of detecting the insects by listening to the noises they make, whether it be displacement, chewing or other noises. These methods are however confronted with some problems, in particular that of random noise likely to distort the result, and, in addition, that of allowing only with difficulty a classification and a densimetric quantification of the insects.
U.S. Pat. No. 4,991,439 discloses an apparatus of this type, using a piezoelectric crystal for detecting the vibrations produced by the granivorous insects, thus allowing, by analyzing the vibrations and by comparing them with known vibrations, determining the species involved. This apparatus is provided according to two embodiments, one that can be used directly in the stocks and the other one by analyzing a sample. The apparatuses are not very reliable from the point of view of the result, because of the random noise. That is why the one operating by means of sampling is preferred, because it allows moving the sample to a quiet place to carry out the analysis of same in the absence of random noise.
The inventor of these apparatuses had already tried to solve the problem arising from the presence of insects in a stock of cereals, and he developed an apparatus for detecting and classifying insects, which is described in U.S. Pat. No. 4,895,025. With this apparatus are recorded the acoustic signals emitted by the insects, said signals are averaged, and the average magnitude according to the frequency is compared with known data corresponding to a type of insects.
This method indeed allows classifying the insects, but, besides the fact that averaging the frequency of signals is not applicable in the case of a practical use in an operational environment, this method has the drawback of not allowing differentiating the insects when the medium being tested contains several families of them. In this case, since it is not possible to accurately classify the various families of insects present in the stocks, it is difficult to contemplate a densimetric quantification.